Normally, the realms of Oblivion and Mundus are divided by a magical barrier that is impassable by both Daedra and mortals. The barrier only exists with the presence of a Septim on the Throne of Cyrodiil and while the Dragonfires are burning. With the last Septim Emperor and his only three heirs dead, however, Dagon was granted a means of access to the mortal world via the Oblivion Gates, and was able to enter onto Nirn along with the Daedric Forces of Destruction to invade Tamriel.

The task of combating the Daedra and the Mythic Dawn fell to Uriel Septim's last surviving son, Martin Septim, as well as his companion known only as "The Hero of Kvatch." Together, Martin and the Hero recovered the Amulet of Kings from Mankar Camoran and the Mythic Dawn. The Amulet was a divine artifact required to rekindle the Dragonfires and restore the barriers between the two worlds.

The Oblivion Crisis devastated all the provinces of Tamriel, but the main theater in which it took place was the Imperial province of Cyrodiil. One city was entirely destroyed and several others left in ruin over the course of the Crisis. Eventually, the Hero recovered the Amulet of Kings from Camoran. Martin defeated Mehrunes Dagon in the Battle of the Imperial City and sealed the Gates of Oblivion, thus ending the Crisis and preventing the Daedra from ever starting another invasion.

The Oblivion Crisis' beginnings were mostly the work of Mankar Camoran, who set the stage for the invasion to commence. Camoran, learning that Dagon planned to attack and needed the magical barriers removed, immediately went to work on a plan to kill the emperor and all his heirs, darkening the Dragonfires for the first time in centuries. The Mythic Dawn, fanatical worshipers of Dagon, acted as the Daedra Lord's earthly agency, and they prepared for his coming, setting the stage for the conflict.

The Mythic Dawn proceeded to launch an attack against the royal family. The emperor's three heirs, Geldall, Enman, and Ebel Septim were slain before any attempt could be made to protect them. Concern immediately rose for the life of Emperor Uriel Septim, and his safety was made top priority. The emperor's organization of bodyguards, the Blades, planned to take Uriel into hiding.

The Blades arranged for Septim to be spirited out of the Imperial City before any assassination attempt could be made. Three Blades, led by Captain Renault, took the emperor away from the palace and down the secret escape route underneath the city's prisons. At the start of the escape route however, they were met by an unlikely obstacle: The cell that housed the secret entrance was mistakenly occupied by a prisoner . As they passed through, Uriel stopped to look at the prisoner, and declared that this person had a great role to play in the coming events. Uriel asked the prisoner to come with them.

The group left the prisons and entered the tunnels beneath the Imperial City. They were unfortunate in that the Mythic Dawn had caught up with them, and they were ambushed repeatedly as they made their escape. During the first ambush, Renault was slain, and Glenroy, another Blade, took point. Glenroy led them further along the passages until at last, in a passage called the Sanctum, the Mythic Dawn were successful in trapping them.

While his guards fought desperately, Uriel turned to the prisoner and handed him the Amulet of Kings off his own neck. The emperor told the prisoner to seek out Jauffre, the Grandmaster of the Blades, who knew the location of the fourth and final heir. "Find him," said the emperor, "and close shut the Jaws of Oblivion!" At that moment, an assassin appeared from a secret entrance and stabbed Uriel Septim VII, killing him. Before the assassin could kill the prisoner, Baurus, the last surviving Blade of the group, came and killed the assassin. Baurus mourned his emperor, then listened to the prisoner's story and sent him off to find Jauffre.

The Mythic Dawn, having succeeded in their first mandate of killing the emperor, now went to work on their second: the opening of the Oblivion Gates that would allow Dagon's forces to attack. This began at the Cyrodiilic city of Kvatch.

The ruins of Kvatch

The assault on Kvatch was both a way of opening the war and a method of killing the final heir. The Mythic Dawn launched the attack at night, opening three Oblivion Gates in front of Kvatch. Dagon's Daedric armies swarmed the city, and yet the three lesser Gates were just the beginning. The Dawn then opened a Great Oblivion Gate, a massive doorway which allowed Dagon's Daedric Siege Crawler to emerge. The Siege Crawler laid waste to Kvatch, setting the city on fire and destroying most of its buildings.

By the end of the assault, few were left alive. Those that survived either managed to escape the city, or were holed up in the Chapel of Akatosh. When it was over and Kvatch was burned, the Daedra occupied the city. The Siege Crawler retreated into the Great Gate, which was closed along with two of the lesser gates. The Mythic Dawn left one gate open in front of the city's gate to prevent any attempt at it being retaken by what was left of the Kvatch guard, led by Captain Savlian Matius.

The Daedra emerge

The heir, however, was not dead. Martin Septim had managed to reach the safety of the Chapel, but was closed in by the Daedra. He was to be rescued by an unlikely hero. The prisoner, who had reached Jauffre near the city of Chorrol , learned that Martin was a priest in Kvatch and set out to recover him. Upon arriving at Kvatch, the prisoner learned the entire story of the attack, and also that Martin was still within the city. The prisoner then decided to work with Captain Matius to lift the siege, but the Oblivion Gate still blocked the way.

The prisoner proceeded into the Gate seeking a way to close it, taking the fight to Oblivion itself. Within the Gate, the prisoner faced a great deal of resistance, but was eventually successful in taking the Gate's Sigil Stone, the thing that anchored it to the mortal world. The Gate was destroyed, and the prisoner went on with Matius and his troops to save the citizens in the Chapel and retake Castle Kvatch. The Daedra suffered their first defeat, and the prisoner found Martin. After a short discussion between them, Martin agreed to follow the prisoner to Weynon Priory and Jauffre.

When he learned of this turn of events, Mankar Camoran made the death of Martin the top priority of the Mythic Dawn. He also launched an assault on Weynon Priory, during which the Amulet of Kings, left in Jauffre's care, was stolen. Weynon Priory was saved from total ruin by the arrival of Martin and the prisoner. Glad of Martin's safety, Jauffre led them both to Cloud Ruler Temple, the fortress and headquarters of the Blades. At this point, the Mythic Dawn began opening Oblivion Gates all across Tamriel. The Oblivion Crisis had begun.

While Martin was getting settled at Cloud Ruler Temple, the prisoner, who became known as the Hero of Kvatch, worked with Baurus in the Imperial City to track down the emperor's assassins, whom Baurus had identified as the Mythic Dawn. The Hero managed to get hold of three of the four copies of the Commentaries on the Mysterium Xarxes, a Mythic Dawn compendium written by Mankar Camoran. The fourth volume was received when Baurus and the Hero proved victorious over Mankar's son and lieutenant, Raven Camoran, in a skirmish in the Imperial City sewers. With those four volumes, the Hero was able to divine the location of the Mythic Dawn's hidden base and Shrine to Mehrunes Dagon.

The Hero infiltrated the Mythic Dawn's secret shrine and there witnessed Mankar Camoran leave the mortal world for his own personal realm of Oblivion, Paradise. Camoran took with him the Amulet of Kings, making it impossible to retrieve through ordinary means. He left behind theMysterium Xarxes, the Holy Book of the Mythic Dawn written by Mehrunes Dagon himself. The Hero took the Xarxes, and was forced to fight his way out of the shrine through many a Mythic Dawn member, including Mankar's daughter and second, Ruma Camoran. With the Xarxes in hand, the Hero returned to Cloud Ruler Temple. Martin began to study the Xarxes for a way to enter Camoran's Paradise and recover the Amulet of Kings.

At the same time, the Mythic Dawn increased their productivity, opening dozens of Oblivion Gates in Cyrodiil and the other provinces, including Gates outside every major city in Tamriel. Dagon's forces began invading in earnest, and the Empire was hard pressed to retaliate.

Though the war's main theater was in Cyrodiil, the other provinces came under attack as well, albeit to a somewhat lesser degree. Not much is known about the Oblivion Crisis in the other provinces, though two of them, Morrowind and Skyrim, were mentioned as being hit hard. Reports from Skyrim indicated massive numbers of Daedra overwhelming the Nords.

Morrowind was also endangered, though more due to infighting than the number of Daedra. The political Great HousesHlaalu and Dres destroyed the House Indoril when Indoril objected to the abolishment of Morrowind's slave trade. The Nords of Skyrim besieged Morrowind's House Redoran. Somewhat ironically, House Telvanni was left to combat the Daedra, but had difficulties closing the Gates. Things were further complicated by the disappearance of two powerful Morrowind figures. Lord Vivec, the last Living God of the Tribunal, mysteriously vanished, and the Nerevarine, the Hero of Morrowind, left on an expedition to Akavir. Ultimately, House Redoran took the largest blow from the Crisis in the Battle of Ald'ruhn.

A different story was in evidence in the province of Black Marsh, however. The Argonians rallied to combat the Daedra, led by a political faction called the An-Xileel. The Argonians defended their land, managing even to take the battle into Oblivion; they struck with such ferocity that the Daedra themselves were forced to close many of their own Gates. Black Marsh was held until the end of the war, bringing the An-Xileel to great prominence.

At the time of the Crisis, Daedra worship had become prominent in the Summerset Isles. A boycott of Imperial goods was already in progress when the Daedra struck. Summerset Isle was plagued by hordes of Daedra, with the Crystal Tower stood as the Altmer's last bastion of hope and thus housed many refugees fleeing the violence. Daedra surrounded the tower and after a few days launched a massacre that led to the slaughter of many elves and the eventual collapse of the Crystal Tower.

At Jauffre's orders, the Hero tracked down two Mythic Dawn spies that were operating out of Bruma. Intelligence gathered from these spies revealed that the Mythic Dawn were aware of Martin's presence at Cloud Ruler Temple and planned to attack Bruma in an effort to kill him. Not long afterward, an Oblivion Gate opened in front of Bruma. The Hero worked with the Bruma guard and Captain Burd to close the Oblivion Gate. Foreseeing that the Mythic Dawn would launch an even greater assault later, Jauffre tasked the Hero with gaining allies from the other cities of Cyrodiil to aid in Bruma's defense.

Martin, meanwhile, was able to discern a method of following Mankar Camoran into his Paradise. In order to create a portal to Paradise, Martin required four objects of power: The Blood of a Daedra Lord, the Blood of a Divine, a Great Welkynd Stone, and lastly a Great Sigil Stone.
The Hero quested for these items, receiving a Daedric artifact from one of Daedric Princes and succeeding where others had failed in cleansing the Shrine of Sancre Tor, a Blades pilgrimage site that had fallen under the thrall of evil. After the Hero got the Great Welkynd Stone from the Ayleid ruins of Miscarcand, Martin revealed that the Great Sigil Stone could only be obtained from a Great Oblivion Gate. Thus, they would have to allow the Mythic Dawn to proceed with their plan to attack Bruma.

The Hero went to all the cities of Cyrodiil, and parlayed troops from closing the Gates that threatened the cities. When at last Bruma was as well defended as it could be, Martin, the Hero, and their Cyrodiilic army marched into battle against the Daedric Forces.

The Second Battle of Bruma was fought then, and was the largest engagement against the Daedra to that point. The soldiers managed to hold off the Daedric charge long enough for the Hero of Kvatch to enter the Great Gate when it opened. Passing through the Oblivion plane beyond, the Hero fought their way to the Sigil Tower and took the Sigil Stone. The Great Gate was closed even as the monstrous Siege Crawler emerged from it. The Siege Crawler was destroyed and the Daedra defeated. The Empire had won a powerful victory and with the final component for the portal ritual in hand, Martin opened a portal to Camoran's Paradise, which the Hero entered in pursuit of the Amulet of Kings.

In Paradise, the Hero had to fight their way through the utopian setting of Camoran's realm. While there, the Hero was constantly confronted by the voice of Mankar Camoran, who told him, among other things, that Dagon's invasion was justified, that Tamriel was in truth another realm of Oblivion, and Dagon sought to liberate it.

In the Forbidden Grotto, the Hero received help in the form of Eldamil, a former lieutenant of Mankar Camoran's that had had a change of heart and now wanted to redeem his past deeds. Together, the Hero and Eldamil escaped the Forbidden Grotto and confronted Mankar Camoran.

The fight against Camoran was difficult, due in part to Ruma and Raven Camoran, who had been resurrected by the power of Paradise. But the Hero was victorious; Mankar Camoran was killed. At his death, Paradise crumbled, and the Hero was transported back to Cloud Ruler Temple with the Amulet of Kings. With the Mythic Dawn defeated, Martin and the Hero turned their sights to the Imperial City, where the Dragonfires were waiting.

In the Imperial City, Martin and the Hero appeared before High Chancellor Ocato, who acted as regent in the Emperor's absence. No sooner had Ocato accepted Martin's claim to the Dragon Throne than a messenger reported the Imperial City was under attack. Oblivion Gates had appeared inside the walls. The capital was burning.

In the fighting that followed, Martin, the Hero, and Ocato made their way to the Temple of the One, where the Dragonfires were. There they beheld the true form of Mehrunes Dagon, come from Oblivion to fight in this last, desperate fight. At first it seemed that hope was gone; Dagon's appearance meant that the barriers between worlds were completely obliterated. But Martin was struck by an idea, and asked the Hero to get him past Dagon. The Hero did so, and in the Temple, Martin bade the Hero farewell, and ran to the Dragonfires.

At that moment, the Temple dome collapsed, and Dagon stepped inside. But Martin shattered the Amulet of Kings, and in a great burst of flame, the Dragonfires transformed him into a flaming dragon: the Avatar of Akatosh. In the Temple of the One, the Dragon and Daedra Lord struggled, Aedra versus Daedra, until at last the Dragon cast Dagon back into Oblivion. At that moment, every Oblivion Gate was closed, and the magical wards against the invading Daedra were restored. In the Temple, the dragon transformed into a giant statue. The Oblivion Crisis was over.

Following the final battle, the Empire was left without an Emperor. Martin had no heir, and so the throne lay empty. But the dire need for an Emperor was gone, the Gates of Oblivion were sealed forever, the Daedra could never attack Tamriel again. The Hero of Kvatch, for the services rendered during the Crisis, was declared the Champion of Cyrodiil.

The Septim Empire, bereft of its god-blessed dynasty, fell in short order after the Crisis' end. Political and cultural wounds left by the war would continue to fester long after the Crisis abated. In particular, the secession of Black Marsh and the subsequent fall of Morrowind left the east in doubt. Tamriel itself would enter a state of disarray not seen since the fall of the Potentates and the Second Empire. But the majority of the empire would be restored under Titus Mede, using conquest to re-establish control.

The main tactic of the Daedra and the Mythic Dawn seemed to be the opening of Oblivion Gates and the attacks on the cities of Tamriel. In the wilderness, Oblivion Gates were opened to allow Daedric creatures to menace the countryside. Dagon and the Mythic Dawn focused their attention on the major cities, using the Daedric Siege Engine to lay waste to them. Currently, the only times known where this has been used were Kvatch and Bruma. Beyond that, the evil forces focused all their power on killing Martin and ending the Septim bloodline. Had they succeeded, the Crisis no doubt would have increased exponentially.

On the other hand, the Empire, having no knowledge of how exactly to defeat the Daedra, did the best they could to hold off the attacks. Though Oblivion Gates were closed, there seemed to be no permanent way of ending the Crisis. The rulers of cities commanded their guards to remain in the walls in the event of an attack, and by the midpoint of the war, the entire Imperial Army was fully committed to the provinces. It was not until Martin's public appearance that victory seemed possible.

While little is known of battles in the other provinces, in Cyrodiil there were many clashes between the Empire and the Daedra with their Mythic Dawn allies. They are as follows (major conflicts in bold):

The Daedra destoyed the Crystal Tower in the Summerset Isles, during this time after taking much of the Isles, the Daedra army in Summerset toppled the tower killing all the defenders and refugees, who were hiding from the Daedra at the time.

The war is fought mainly at the cities. In the wilderness, Daedra appear but they do not attack until approached.

Not much is known about the effect of the war in the other provinces. NPC dialogue and accounts from the novel The Infernal City, set roughly forty years after the Crisis, and Skyrim set two hundred years after the crisis, give testament to what is known. The book series Rising Threat shed light on the events in Summerset.